- In decapod crustaceans, a number of neurohormones regulating a variety of physiological processes, including reproduction, are to be found in the X-organ-sinus gland complex of the eyestalk. Bilateral eyestalk ablation was thus performed in mature males of the Australian red claw crayfish Cherax quadricarinatus with the aim of studying the role of eyestalk-borne hormones on spermatogenic activity in the testis and on the androgenic gland (AG). The latter gland controls the differentiation and functioning of male sexual characteristics in crustaceans. Eyestalk ablation caused hypertrophy of the AG, as indicated by an increase in gland weight (3.9 +/- 0.44 mg vs < 0.1mg in intact males) and by overexpression of AG polypeptides. In the testes of eyestalk-ablated males, empty spermatogenic lobules were common, while lobules containing primary spermatocytes were infrequent. These findings were reflected in decreased amounts of DNA in these testes and a consequent increase in the relative weights of the sperm ducts. Since it was found that eyestalk ablation affected both the AG and the reproductive system, in vitro experiments were conducted to study the direct effects of the sinus gland on the AG and testes and of the AG on the testes. Sinus gland extracts inhibited by 30% the incorporation of radiolabeled amino acids into AG polypeptides and almost totally inhibited the secretion of radiolabeled AG polypeptides into the culture medium. However, sinus gland extracts had no significant effects on testicular tissue. On the other hand, AG extracts affected the in vitro phosphorylation of a testicular polypeptide (of 28 kDa), in a time- and dose-dependent manner, suggesting a direct effect of AG-borne hormones on the testes. The above findings, together with the evidence for direct inhibition by the sinus gland on the AG, suggest an endocrine axis-like relationship between the sinus gland, the AG, and the male reproductive system in decapod crustaceans.